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Full text of "Treatise On Applied Analytical Chemistry(Vol-1)"

SOAPS

459

stances such as alkali carbonates, chlorides, sulphates, silicates and berates
kaolin, talc, starch, sugars, glycerine, alcohol, etc, essential oils (in scented
soaps) or antiseptic and medicinal substances (in medicinal soaps)

The purest soaps are the so-called curd soaps, obtained by precipitation
with common salt, and those prepared from the free fatty acids. Such
soaps are neutral or only slightly alkaline, and this group includes, for
instance, toilet soaps in general, so-called Marseilles white soap and certain
mottled soaps.

So-called '' cold " soaps, obtained by saponifying fats with an alkaline
lye without adding common salt, are usually very alkaline ; they contain
the glycerine and all the impurities of the fats and alkali used" Potash
soaps belong to this class.

Soaps arc analysed to ascertain their composition and to see if they
are suited to do finite uses. Analysis includes mainly determinations of
the water, total fat, alkali, resin, glycerine, etc.

1.   Sampling.- -The various determinations require at least 100 grams
of soap, which is stored in a well-dried and closed glass jar.

With soap in small pieces or cakes, these are cut into at least four parts
(lengthwise and crosswise), from which thin shavings are taken and cut
up finely, the whole being thoroughly mixed and a portion taken for analysis.

With soap in blocks or large rectangular pieces, two triangular prisms
with their bases on two opposite sides are taken so as to represent propor-
tionately the dried outer part and the more hydrated inner part; these
prisms are rapidly cut up, well mixed and the sample to be analysed then
taken.

Soft or powdered soaps are well mixed with a spatula or in a mortar and
the sample then taken.

2.  Water.—In a platinum dish, tared with a glass rod, 5-8 grams of
the soap are weighed, the dish being then heated in an oven first at 60-70°
and then at 100-105° until of constant weight, the mass being stirred from
time to time with the. rod.    Loss of weight represents water.

For soft soaps or others containing a large proportion of water, a certain
amount of siliceous sand or ground pumice, previously ignited, is tared
in the dish with the glass rod.1

3.  Total   Fat.—20   grams   of   the   soap,   dissolved  in  water,   are
decomposed by excess of dilute sulphuric acid (i: 3) or of normal sulphuric
acid (see also 4), the solution shaken with 100 c.c. of petroleum ether, b.pt.
not above 65°, and the acid liquid separated and again shaken with 100
c.c. of petroleum ether.   The two petroleum extracts are united, washed
with water, filtered (if necessary) into a tared dish and evaporated at a
low temperature, the residue being dried in an oven at about 110° to constant
weight.

This gives the total fat, which, besides the fatty acids and resin acids
constituting normal soap, may contain also neutral fat and unsaponifiable
substances ; it is therefore examined further (see below: 5, 8 and 11).

* If the soap contains alcohol, hydrocarbons or other volatile substances these
are vaporised with the water; in such cases the water is determined by difference
after the other components of the soap have been determined.ciation of Leather Trade Chemists, iurin,